Question

here is my problem: I have a script (let’s call it comet.php) whic is requsted by an AJAX client script and wait for a change to happen like this:

while(no_changes){
    usleep(100000);
    //check for changes
}

I don’t like this too much, it’s not very scalable and it’s (imho) “bad practice”
I would like to improve this behaviour with a semaphore(?) or anyway concurrent programming
technique. Can you please give me some tips on how to handle this? (I know, it’s not a short answer, but a starting point would be enough.)

Edit: what about LibEvent?

Answer 1

You can solve this problem using ZeroMQ.

ZeroMQ is a library that provides supercharged sockets for plugging things (threads, processes and even separate machines) together.

I assume you’re trying to push data from the server to the client. Well, a good way to do that is using the EventSource API (polyfills available).

client.js

Connects to stream.php through EventSource.

var stream = new EventSource('stream.php');

stream.addEventListener('debug', function (event) {
    var data = JSON.parse(event.data);
    console.log([event.type, data]);
});

stream.addEventListener('message', function (event) {
    var data = JSON.parse(event.data);
    console.log([event.type, data]);
});

router.php

This is a long-running process that listens for incoming messages and sends them out to anyone listening.

<?php

$context = new ZMQContext();

$pull = $context->getSocket(ZMQ::SOCKET_PULL);
$pull->bind("tcp://*:5555");

$pub = $context->getSocket(ZMQ::SOCKET_PUB);
$pub->bind("tcp://*:5556");

while (true) {
    $msg = $pull->recv();
    echo "publishing received message $msg\n";
    $pub->send($msg);
}

stream.php

Every user connecting to the site gets his own stream.php. This script is long-running and waits for any messages from the router. Once it gets a new message, it will output this message in EventSource format.

<?php

$context = new ZMQContext();

$sock = $context->getSocket(ZMQ::SOCKET_SUB);
$sock->setSockOpt(ZMQ::SOCKOPT_SUBSCRIBE, "");
$sock->connect("tcp://127.0.0.1:5556");

set_time_limit(0);
ini_set('memory_limit', '512M');

header("Content-Type: text/event-stream");
header("Cache-Control: no-cache");

while (true) {
    $msg = $sock->recv();
    $event = json_decode($msg, true);
    if (isset($event['type'])) {
        echo "event: {$event['type']}\n";
    }
    $data = json_encode($event['data']);
    echo "data: $data\n\n";
    ob_flush();
    flush();
}

To send messages to all users, just send them to the router. The router will then distribute that message to all listening streams. Here’s an example:

<?php

$context = new ZMQContext();

$sock = $context->getSocket(ZMQ::SOCKET_PUSH);
$sock->connect("tcp://127.0.0.1:5555");

$msg = json_encode(array('type' => 'debug', 'data' => array('foo', 'bar', 'baz')));
$sock->send($msg);

$msg = json_encode(array('data' => array('foo', 'bar', 'baz')));
$sock->send($msg);

This should prove that you do not need node.js to do realtime programming. PHP can handle it just fine.

Apart from that, socket.io is a really nice way of doing this. And you could connect to socket.io to your PHP code via ZeroMQ easily.

See also

• ZeroMQ
• ZeroMQ PHP Bindings
• ZeroMQ is the Answer – Ian Barber (Video)
• socket.io